Mathematically inclined
geographers are ashamed to
admit that no formal definition
of what a river is exists. Because
rivers are well-described as
fractals, establishing length for
even surface rivers is difficult.
One can say a tidal bore or aegir
exists when the flood
(incoming) tide is funnelled into
a river mouth and is powerful
enough to reverse the river flow
to the delight of surfers.
Currently, there are over 400 rivers in the world with tidal bores.  Data is sparse, but the most extreme appears to
be the Qiantang river in southeast China. American surfers Mark Healy and Greg Long reported their adventures
on the Silver Dragon at in an entry dated September 18, 2008. So far, no equation predicting bore
height and speed, nor any indication if tsunami waves in straits and rivers are similar to tidal bores.